Light emitting device with reflective sidewall

ABSTRACT

Embodiments of the invention include a light emitting device including a substrate and a semiconductor structure including a light emitting layer. A first reflective layer surrounds the light emitting device. A wavelength converting element is disposed over the light emitting device. A second reflective layer is disposed adjacent a first sidewall of the wavelength converting element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/105,096 titled “LIGHT EMITTING DEVICE WITH REFLECTIVE SIDEWALL” andfiled Jun. 6, 2006 as a § 371 application of International ApplicationNo. PCT/IB2015/050025, which was filed on Jan. 2, 2015 and also titled“LIGHT EMITTING DEVICE WITH REFLECTIVE SIDEWALL.” InternationalApplication No. PCT/IB2015/050025 claims priority to U.S. ProvisionalApplication No. 61/925,328 filed Jan. 9, 2014. U.S. patent applicationSer. No. 15/105,096, International Application No. PCT/IB2015/050025,and U.S. Provisional Application No. 61/925,328 are each incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to a light emitting device with a reflectivesidewall.

BACKGROUND

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,III-nitride, or other suitable substrate by metal-organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxialtechniques. The stack often includes one or more n-type layers dopedwith, for example, Si, formed over the substrate, one or more lightemitting layers in an active region formed over the n-type layer orlayers, and one or more p-type layers doped with, for example, Mg,formed over the active region. Electrical contacts are formed on the n-and p-type regions.

LEDs where a majority of light is extracted through the top of thedevice may be formed by molding a reflective material around the sidesof the device, to prevent light from escaping from the sides. Molding isillustrated in FIG. 1, which is described in more detail in US2011/0018017. FIG. 1 illustrates the submount wafer 360 and LEDs 100attached to the submount. Lines are drawn on the wafer 360 illustratingwhere the wafer 360 will be later sawed or broken for singulation.

A mold 400, also known as a chase, has indentions 420 that arepreferably shallow to ensure that the tops of the LEDs contact or comevery close to the flat bottom surface of each indention 420. Theindentions 420 are slightly wider than the LEDs 100, where thedifference will be the thickness of the molded material covering thesides of the LEDs 100. A viscous mixture 440 of silicone and TiO₂ isprecisely dispensed over the mold 400 to fill the indentions 420 andalso create a thin layer between the indentions 420. The submount wafer360 and mold 400 are brought together under pressure so that the LEDs100 are immersed in the mixture 440. When the tops of the LEDs 100 arejust touching the bottoms of the indentations 420, pressure ismaintained and the silicone is cured, such as by heating. The wafer 360and mold 400 are then separated, and the hardened silicone/TiO₂ 460 maybe further cured by heating or UV. The submount wafer 360 is thensingulated along the lines by sawing or breaking.

The relatively thick layer of silicone/TiO₂ covering the sides of theLED 100 reflects substantially all of the LED side light (e.g., at least75%). After any reflection off the silicone/TiO₂ sidewall, a portion ofthe reflected light will ultimately exit through the top surface of theLED 100 (the top surfaces of the LEDs 100 are facing down in theorientation illustrated in FIG. 1).

SUMMARY

It is an object of the invention to provide a light emitting device witha reflective sidewall, with flexibility in the placement of thereflective material.

Embodiments of the invention include a light emitting device including asubstrate and a semiconductor structure including a light emittinglayer. A first reflective layer surrounds the light emitting device. Awavelength converting element is disposed over the light emittingdevice. A second reflective layer is disposed adjacent a first sidewallof the wavelength converting element.

A method according to embodiments of the invention includes attaching alight emitting device to a mount. A first reflective layer is formedadjacent a sidewall of the light emitting device. A wavelengthconverting element is disposed over the light emitting device. A secondreflective layer is formed adjacent a sidewall of the wavelengthconverting element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates molding reflective material over an LED.

FIG. 2 illustrates one example of a III-nitride LED.

FIG. 3 illustrates an automotive headlamp.

FIGS. 4 and 5 are cross sectional views of an LED in the headlamp ofFIG. 3.

FIG. 6 illustrates LEDs attached to a mount.

FIG. 7 illustrates the structure of FIG. 6 after forming a reflectivelayer over the LEDs.

FIG. 8 illustrates the structure of FIG. 7 after removing the reflectivematerial over the tops of the LEDs.

FIG. 9 illustrates the structure of FIG. 8 after disposing wavelengthconverting layers over the LEDs.

FIG. 10 illustrates the structure of FIG. 9 after forming an additionalreflective layer over a side of the wavelength converting layer.

DETAILED DESCRIPTION

In embodiments of the invention, a reflective material is molded over alight emitting device, a wavelength converting element is disposed overthe light emitting device, then an additional reflective layer is formedover at least a portion of the wavelength converting element.Embodiments of the present invention offer flexibility in the locationof the reflective material, which is not available in the methodillustrated in FIG. 1.

FIG. 3 illustrates an example of a head lamp for an automobile. Theheadlamp 30 includes five light sources 34. Though light sources 34 areoften used, for example, as low beam head lights and/or as day timerunning lights, light sources 34 may be used for any appropriatepurpose. Light sources 34 are wavelength converted III-nitride LEDs inembodiments of the invention. Light sources 34 are disposed on asubstrate 31. Bond pads 32 on the substrate are used to form electricalconnections to the light sources 34. Light sources 34 may beelectrically connected to bond pads 32 through traces or conductive viasformed within or on the surface of substrate 31 (not shown in FIG. 3).

Though in the examples described herein the semiconductor light emittingdevices are III-nitride LEDs that emit blue or UV light, semiconductorlight emitting devices besides LEDs such as laser diodes andsemiconductor light emitting devices made from other materials systemssuch as other III-V materials, III-phosphide, III-arsenide, II-VImaterials, ZnO, or Si-based materials may be used.

FIG. 2 illustrates a III-nitride LED 1 that may be used in embodimentsof the present invention. Any suitable semiconductor light emittingdevice may be used and embodiments of the invention are not limited tothe device illustrated in FIG. 2. The device of FIG. 2 is formed bygrowing a III-nitride semiconductor structure on a growth substrate 10as is known in the art. The growth substrate is often sapphire but maybe any suitable substrate such as, for example, SiC, Si, GaN, or acomposite substrate. A surface of the growth substrate on which theIII-nitride semiconductor structure is grown may be patterned,roughened, or textured before growth, which may improve light extractionfrom the device. A surface of the growth substrate opposite the growthsurface (i.e. the surface through which a majority of light is extractedin a flip chip configuration) may be patterned, roughened or texturedbefore or after growth, which may improve light extraction from thedevice.

The semiconductor structure includes a light emitting or active regionsandwiched between n- and p-type regions. An n-type region 16 may begrown first and may include multiple layers of different compositionsand dopant concentration including, for example, preparation layers suchas buffer layers or nucleation layers, and/or layers designed tofacilitate removal of the growth substrate, which may be n-type or notintentionally doped, and n- or even p-type device layers designed forparticular optical, material, or electrical properties desirable for thelight emitting region to efficiently emit light. A light emitting oractive region 18 is grown over the n-type region. Examples of suitablelight emitting regions include a single thick or thin light emittinglayer, or a multiple quantum well light emitting region includingmultiple thin or thick light emitting layers separated by barrierlayers. A p-type region 20 may then be grown over the light emittingregion. Like the n-type region, the p-type region may include multiplelayers of different composition, thickness, and dopant concentration,including layers that are not intentionally doped, or n-type layers.

After growth, a p-contact is formed on the surface of the p-type region.The p-contact 21 often includes multiple conductive layers such as areflective metal and a guard metal which may prevent or reduceelectromigration of the reflective metal. The reflective metal is oftensilver but any suitable material or materials may be used. After formingthe p-contact 21, a portion of the p-contact 21, the p-type region 20,and the active region 18 is removed to expose a portion of the n-typeregion 16 on which an n-contact 22 is formed. The n- and p-contacts 22and 21 are electrically isolated from each other by a gap 25 which maybe filled with a dielectric such as an oxide of silicon or any othersuitable material. Multiple n-contact vias may be formed; the n- andp-contacts 22 and 21 are not limited to the arrangement illustrated inFIG. 2. The n- and p-contacts may be redistributed to form bond padswith a dielectric/metal stack, as is known in the art.

In order to form electrical connections to the LED 1, one or moreinterconnects 26 and 28 are formed on or electrically connected to then-and p-contacts 22 and 21. Interconnect 26 is electrically connected ton-contact 22 in FIG. 2. Interconnect 28 is electrically connected top-contact 21. Interconnects 26 and 28 are electrically isolated from then- and p-contacts 22 and 21 and from each other by dielectric layer 24and gap 27. Interconnects 26 and 28 may be, for example, solder, studbumps, gold layers, or any other suitable structure.

Many individual LEDs are formed on a single wafer then diced from awafer of devices. The semiconductor structure, the n-and p-contacts 22and 21, and the interconnects 26 and 28 excluding the substrate 10 arerepresented in the following figures by block 12.

Returning to FIG. 3, the LEDs 34 are located at the top 36 of the headlamp and are therefore the closest light sources to the driver.Accordingly, it is desirable to direct light away from the edge of theLEDs 34 that is closest to the top 36, in order to avoid causing glarevisible to the driver. A reflector layer is formed to direct light awayfrom the top 36 of head lamp 30. FIG. 4 illustrates a cross section ofan LED 34 taken along axis 38. FIG. 5 illustrates a cross section of anLED 34 taken along axis 40.

In FIGS. 4 and 5, an LED 1, which may be the device illustrated in FIG.2 or any other suitable device, is electrically and physically connectedto a mount 42A. LED 1 is attached to mount 42A with the semiconductorstructure and contacts facing down, such that light is extracted throughthe substrate 10. A reflective material 44 is formed adjacent the foursidewalls of LED 1. Reflective material 44 may be, for example,reflective particles such as TiO₂ disposed in a transparent material,such as silicone or silicone molding compound. The reflective material44 extends to the top of the substrate 10 of LED 1. The reflectivematerial 44 is at least 90% reflective in some embodiments and at least95% reflective in some embodiments.

A wavelength converting layer 46 is disposed over the substrate 10 ofLED 1. Wavelength converting layer 46 may be attached to substrate 10 bygluing, as described below.

In some embodiments, the top of reflective material 44 is below the topsurface of the substrate 10, to reduce or eliminate interaction betweenreflective material 44 and the glue, which can cause cracking of thereflective material 44 or other reliability problems.

An additional reflective material 48 is disposed over a side of thewavelength converting layer 46, as illustrated in FIG. 4. The additionalreflective material 48 may reduce the amount of light emitted from atleast one side of wavelength converting layer 46. The additionalreflective material 48 may be positioned adjacent the top 36 of headlamp30, in order to reduce glare apparent to the driver. The additionalreflective material 48 is not formed on the other sidewalls of thewavelength converting layer which are not proximate the top of headlamp30. The additional reflective material 48 is at least 90% reflective insome embodiments and at least 95% reflective in some embodiments. In thealternative, additional reflective material 48 may be part of reflectivematerial 44. For example, reflective material 44 may be formed higherthan substrate 10 then selectively removed.

The LEDs illustrated in FIGS. 4 and 5 may be formed by the processillustrated below in FIGS. 6, 7, 8, 9, and 10. Although only two LEDs 1Aand 1B are shown, many LEDs or in some cases only a single LED may beprocessed together on a single mount wafer 42.

In FIG. 6, LEDs 1A, 1B are attached to a tile or a mount wafer 42. (Thewafer of mounts is shown in FIGS. 6-10 as a single, continuousstructure. After the processing illustrated in FIGS. 6-10, the wafer isdiced for example into individual mounts 42A, 42B etc. eachcorresponding to a single LED, as illustrated in FIG. 10.) Mount wafer42 may be, for example, silicon, metal, ceramic, or any other suitablematerial. Mount wafer 42 may include integrated circuitry. LEDs 1 may beattached to mount wafer 42 by gold stud bumps, gold layers, solder,thermosonic bonding, ultrasonic bonding, or any other suitable method ormaterial. In some embodiments, LEDs 1A, 1B are attached to metal contactpads disposed on the top surface of mount wafer 42. These contact padsare connected to metal contact pads on the bottom surface of mount wafer42, for example by traces formed within and/or on the surface of themount. The bottom contact pads may be used to connect the LEDs 1A, 1B toany suitable structure such as a PC board or other substrate thatsupports the light sources 32 and 34 in headlamp 30 of FIG. 3. Top andbottom contacts on the mount are known in the art and not shown in FIG.6.

In some embodiments, an underfill is injected between LEDs 1 and mountwafer 42. Underfill may support LEDs 1 and/or seal the LEDs 1 againstcontaminants. Excessive underfill may be removed by any suitabletechnique such as microbead blasting. The use and removal of underfillis known in the art.

In FIG. 7, a reflective material 44 is molded over LEDs 1A, 1B. Forexample, a mold may be disposed over a wafer 42 of mounts on which LEDs1A, 1B have been mounted. The mold may include indentations thatcorrespond to the shape of the LEDs 1A, 1B, though it need not. Amolding compound, which is a viscous mixture of a matrix material (oftensilicone, epoxy, or glass, but any suitable material may be used) andreflective particles (often TiO₂ but any suitable material may be used),is disposed over the mold to fill the mold. The wafer of mounts 42 andLEDs 1A, 1B and the mold are brought together under pressure so that theLEDs 1A, 1B are immersed in the molding compound. A vacuum may becreated between the mold and the mount wafer. The molding compound iscured, such as by heating. The mount wafer and mold are then separated.The hardened molding compound may be further cured, for example byheating or exposing to ultraviolet radiation. After curing, the moldingcompound is generally reflective, white, and opaque.

Though in the above description, the transparent material is a thermosetmolding compound that is molded, any material that supports thereflective particles and can be disposed around the LEDs 1A, 1B may beused. In some embodiments, rather than molding compound, a sol gelmaterial is used. In such embodiments, a mixture of reflective particlesand sol gel liquid may be dispensed over the LEDs 1A, 1B, then water isevaporated from the sol gel liquid, leaving a silicate network that isessentially a glass with reflective particles embedded in the silicatenetwork.

In some embodiments, a material with a high thermal conductivity; forexample, with a higher thermal conductivity than the transparentmaterial and/or the reflective particles, may be added to the mixture.For example, the material with high thermal conductivity may have athermal conductivity higher than that of common silicone materials,which may have a thermal conductivity around 0.1-0.2 W/mK.

In FIG. 7, reflective material 50 may be disposed over the tops of LEDs1A, 1B. In FIG. 8, the reflective material 50 over the LEDs is removed.The excess reflective material 50 may be removed by wet beat blasting orany other suitable technique. After removing excessive reflectivematerial, the top surface of the reflective material 44 may be at thesame level as the top surface of LEDs 1A, 1B, as illustrated in FIG. 8,though this is not required: the top surface of the reflective material44 may be higher or lower than the top surface of the LEDs 1A, 1B.

In FIG. 9, wavelength converting elements 46 are disposed over the LEDs1A, 1B. Wavelength converting elements may be, in some embodiments,pre-formed elements that are attached to the LEDs 1A, 1B by a layer ofadhesive 45. Adhesive 45 is often silicone but any suitable material maybe used. Examples of pre-formed wavelength converting elements 46include powder phosphors that are sintered or otherwise formed intoceramic sheets, then singulated into individual platelets sized for asingle LED, and powder phosphors that are disposed in transparentmaterial such as silicone or glass that is rolled, cast, or otherwiseformed into a sheet, then singulated into individual platelets.Wavelength converting elements 46 need not be pre-formed elements insome embodiments, and may be formed by any suitable technique includinglamination, molding, spray-coating, spin-coating, or screen-printing.

The wavelength converting elements 46 include a wavelength convertingmaterial which may be, for example, conventional phosphors, organicphosphors, quantum dots, organic semiconductors, II-VI or III-Vsemiconductors, II-VI or III-V semiconductor quantum dots ornanocrystals, dyes, polymers, or other materials that luminesce. Thewavelength converting material absorbs light emitted by the LED andemits light of one or more different wavelengths. Unconverted lightemitted by the LED is often part of the final spectrum of lightextracted from the structure, though it need not be. Examples of commoncombinations include a blue-emitting LED combined with a yellow-emittingwavelength converting material, a blue-emitting LED combined with green-and red-emitting wavelength converting materials, a UV-emitting LEDcombined with blue- and yellow-emitting wavelength converting materials,and a UV-emitting LED combined with blue-, green-, and red-emittingwavelength converting materials. Wavelength converting materialsemitting other colors of light may be added to tailor the spectrum oflight extracted from the structure.

As illustrated in FIG. 9, the top surfaces of wavelength convertingelements 46 are often higher than the top surface of reflective material44. In some embodiments, reflective material 44 does not cover anyportion of a sidewall of the wavelength converting element 46.

In FIG. 10, an additional reflective material 48 is disposed on at leastone side of the wavelength converting element of LEDs 1A, 1B. In theheadlamp illustrated in FIG. 3, the LEDs 34 (which are, for example,singulated LEDs 1A, 1B) have an additional quantity of reflectivematerial 48 disposed on one side of the LEDs 34, the side of the LEDs 34oriented toward the top 36 of the headlamp. For other applications,additional reflective material 48 may be disposed on more than one sideof the LEDs 1A, 1B.

Additional reflective material 48 may be, for example, a reflectivematerial such as TiO₂ particles disposed in a transparent material suchas silicone. Any suitable reflective material and transparent materialmay be used. Additional reflective material 48 may be formed by anysuitable technique such as, for example, jet dispensing over thesidewall of wavelength converting element 46, then curing the additionalreflective material 48 for example by heating or any other suitabletechnique. In some embodiments, additional reflective material 48 andreflective layer 44 are formed by different techniques. The additionalreflective material 48 may be the same material as reflective layer 44,or a different material.

In the alternative, additional reflective material 48 may be a portionof reflective layer 44. Removal as shown in FIG. 8 may be via a maskingstep such that additional reflective material 48 remains on one moresides of the LEDs 1A, 1B.

In the device illustrated in FIG. 10, the additional reflective material48 is arranged such that a majority of light emitted toward the sidewall56 of wavelength converting element 46 is reflected back into thewavelength converting element, while a majority of light emitted towardthe sidewall 54 of wavelength converting element 46 is extracted fromthe wavelength converting element. Additional reflective material 48need not be limited to a single surface of the wavelength convertingelement 46 and may be formed on any surface as appropriate to theapplication for which the LEDs 1A, 1B is to be used.

Individual LEDs 1A, 1B or groups of LEDs 1A, 1B may then be separatedfrom the wafer, for example by cutting through the mount wafer 42 andreflective material 44 in regions 52.

The LEDs 1A, 1B are then ready to be incorporated into headlamp 30 ofFIG. 3 or any other suitable device. Note that FIGS. 6, 7, 8, 9, and 10illustrate manufacturing the devices 34 illustrated in FIGS. 3, 4, and5. Accordingly, the orientation of the additional reflective material 48on the devices is different on the manufacturing wafer illustrated inFIG. 10 than in the headlamp 30 of FIG. 3.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

What is claimed is:
 1. A device comprising: a light emitting devicecomprising a semiconductor light emitting layer, side walls, and a topsurface; a wavelength converting element comprising side walls, a bottomsurface, and a top surface, the wavelength converting element disposedover the top surface of the light emitting device with the bottomsurface of the wavelength converting element facing the top surface ofthe light emitting device; a first reflective layer contacting andconforming to side walls of the light emitting device and not extendingalong the side walls of the wavelength converting element as far as thetop surface of the wavelength converting element; and a secondreflective layer disposed adjacent a first side wall of the wavelengthconverting element.
 2. The device of claim 1, wherein in operation amajority of light exiting the wavelength converting element exitsthrough the top surface of the wavelength converting element, and amajority of light impinging on the second reflective layer from withinthe wavelength converting element is reflected back into the wavelengthconverting element.
 3. The device of claim 1, wherein the firstreflective layer does not extend along the side walls of the lightemitting device as far at the top surface of the light emitting device.4. The device of claim 1, wherein the first reflective layer does notextend along the side walls of the light emitting device beyond the topsurface of the light emitting device.
 5. The device of claim 1, whereinthe wavelength converting element is formed separately from the lightemitting device and attached to the light emitting device by anadhesive.
 6. The device of claim 1, wherein the wavelength convertingelement comprises a second side wall positioned oppositely from thefirst sidewall of the wavelength converting element and not covered by areflective layer.
 7. The device of claim 6, wherein the secondreflective layer is arranged such that in operation a majority of lightemitted toward the first sidewall is reflected into the wavelengthconverting element and a majority of light emitted toward the secondsidewall of the wavelength converting element is extracted from thewavelength converting element.
 8. The device of claim 1, wherein thefirst reflective layer is at least 95% reflective.
 9. The device ofclaim 1, wherein the first reflective layer comprises reflectiveparticles disposed in a transparent material.
 10. The device of claim 1,wherein the second reflective layer is disposed adjacent only a singlesidewall of the wavelength converting element.
 11. The device of claim1, wherein the second reflective layer is at least 95% reflective. 12.The device of claim 1, wherein: the first reflective layer does notextend along the side walls of the light emitting device beyond the topsurface of the light emitting device; and the wavelength convertingelement comprises a second side wall positioned oppositely from thefirst sidewall of the wavelength converting element and not covered by areflective layer.
 13. The device of claim 12, wherein the secondreflective layer is disposed adjacent only the first sidewall of thewavelength converting element.
 14. The device of claim 12, wherein thefirst reflective layer and the second reflective layer each comprisesreflective particles disposed in a transparent material.
 15. The deviceof claim 12, wherein: the second reflective layer is disposed adjacentonly the first sidewall of the wavelength converting element; and thefirst reflective layer and the second reflective layer each comprisesreflective particles disposed in a transparent material.
 16. A methodcomprising: attaching a light emitting device to a mount; disposing awavelength converting element over the light emitting device with abottom surface of the wavelength converting element facing the topsurface of the light emitting device; forming a first reflective layercontacting and conforming to side walls of the light emitting device andnot extending along side walls of the wavelength converting element asfar as the top surface of the wavelength converting element; and forminga second reflective layer disposed adjacent a first side wall of thewavelength converting element.
 17. The method of claim 16, comprisingforming the second reflective layer after forming the first reflectivelayer.
 18. The method of claim 16, comprising forming the firstreflective layer by molding and forming the second reflective layer byjet dispensing.
 19. The method of claim 16, comprising forming the firstreflective layer and the second reflective layer by differenttechniques.
 20. The method of claim 16, wherein the wavelengthconverting element is pre-formed, comprising gluing the wavelengthconverting element to the top surface of the light emitting device.